Wireless power was proposed a century ago by Nicolas Tesla. FIG. 1 depicts one of Tesla's experiments showing a 2-resonator system. However, for the first half of the 20th century, no serious application was found because of the relatively poor energy efficiency of 2-coil systems as transmission distances increased. In the 1960's, wireless power transfer research regained interest in medical implant applications (see References 1 to 4 listed below). Recently, 4-coil systems (see References 5 and 6 listed below), as shown in FIG. 2, and domino systems with relay resonators (see References 7 to 10 listed below), as shown in FIG. 3, have been proposed.
It has been pointed out in a recent critical review (see Reference 11 listed below) of wireless power transfer that non-radiative wireless power transfer technologies can be classified as one of two approaches. The first approach is the maximum energy efficiency (MEE) method and the second approach is the maximum power transfer (MPT) method.
The MEE method is based on the near-field magnetic coupling of the coils or coil-resonators and does not require any impedance matching between the driving source and the driven system. This approach does not have the restriction of limiting the overall system energy efficiency to be not higher than 50%. References 9 and 10 listed below fall under this category. This MEE approach has been the common practice in the design of transformers and switched mode power supplies. This approach is suitable for operating frequencies not higher than several mega-hertz and is very suitable for short-range wireless power transfer.
The MPT method, on the other hand, requires the impedance matching of the driving source and the system impedance. It is also called the magnetic resonance method. References 5 to 7 listed below fall under this category. This approach suffers an inherent limitation in that the overall system energy efficiency can never exceed 50%, which is a feature of the maximum power transfer theorem. However, an extended wireless power transmission distance can be achieved, at the expense of energy efficiency. This approach has been used in high-frequency communication circuits and antenna designs. Impedance matching is a key characteristic in this approach.
Up until the present time, the majority of the non-radiative wireless power systems have the power flow either in one direction (i.e. 1-dimensional power flow) or two directions on the same plane (i.e. 2-dimensional power flow). However, two recent reports explore the possibility of omni-directional wireless power (i.e. 3-dimensional power flow). The authors of Reference 12 listed below suggest the use of orthogonal coils to reduce the effect of small mutual inductance when the receiver coil is perpendicular to one of the transmitter coils. They consider the open-ended coils as antennas, and use the parasitic coil inductance and capacitance to form an equivalent LC circuit. Since they consider the coils as antennas, their design approach (based on MPT) suffers the following limitations:                1. The length of the wire used to implement the resonant circuit is comparable to the wavelength at the resonant frequency. Both the transmitter and receiver coils are one quarter of the wavelength at the resonant frequency. This approach is therefore dimension dependent and is restrictive in terms of the relative sizes of the transmitter and receiver coils.        2. Due to the usually low parasitic capacitance in open-ended coils, the resonant frequency and therefore the operating frequency is usually high. High-frequency AC power sources are usually more expensive than low-frequency AC power sources.        
The authors of Reference 12 listed below drove the two separate orthogonal coils with the same AC current (i.e. the two separate coils are connected in series).
This is why they could demonstrate that the receiver coil can pick up maximum power at an angle of 45° between the two orthogonal transmitter coils. This result is reasonable because at 45°, the vectorial sum of the two co-axial magnetic field vectors from the two orthogonal coils is at a maximum if the two coil currents are identical. They also suggested extending the concept to a 3-dimensional structure based on 3 separate orthogonal coils that are connected in series and fed by the same current.
In Reference 13 listed below, a 3-coil receiver structure with 3 orthogonal open-ended coils was placed inside a similar but larger 3-coil transmitter structure also with open-ended coils (see FIG. 5). Again, the 3 orthogonal transmitter coils were connected in series and driven with the same AC current. It was demonstrated that wireless power transfer to the 3-coil receiver unit can be achieved regardless of the orientation of the receiver unit inside the transmitter structure. However, this orientation-insensitive feature is only possible if the receiver has 3 orthogonal coils. For RFID tag applications, it is more likely to have a single planar coil in the RFID tag as a receiver coil. Therefore, the approach in Reference 13 is not suitable for a single-coil receiver.
In summary, the magnetic resonance techniques used in both References 12 and 13 listed below are based on impedance matching and adopt the MPT method. According to Reference 11 listed below, the system energy efficiency will never exceed 50%. The use of the same current in the orthogonal coils also does not generate magnetic field vectors that point in all directions in a 3-dimensional (3D) manner, which is an essential feature for true omni-directional wireless power transfer.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.